1. Field of the Invention
This invention relates to a charging member and a charging device suitably used in an image forming apparatus of the electrophotographic type to charge a member to be charged such as a photosensitive member.
2. Related Background Art
In an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, a corona charger (corona discharger) has heretofore been often used as a charging device for uniformly charging (and removing charges) an image bearing member (a member to be charged) such as an electrophotographic photosensitive member or an electrostatic recording dielectric member to a required polarity and potential.
The corona charger is a noncontact type charging device and is provided, for example, with a discharge electrode such as a wire electrode and a shield electrode surrounding the discharge electrode, and a discharge opening portion is disposed in an opposed noncontact relationship with the image bearing member which is a member to be charged, and a high voltage is applied to the discharge electrode and the shield electrode to thereby create a discharge current (corona shower), and the surface of the image bearing member is exposed to the discharge current to thereby charge the surface of the image bearing member to a predetermined polarity and potential.
[Contact Charging]
Recently, a charging device of the contact type (contact charging device) for charging a member to be charged by the charging member to which a voltage has been applied as previously described being brought into contact with the member to be charged has been put into practical use owing to the advantages of low ozone, low electric power, etc. as compared with the corona charger.
The contact charging device is such that an electrically conductive charging member of the roller type (charging roller), the fur brush type, the magnetic brush type, the blade type or the like is brought into contact with a member to be charged such as an image bearing member and a predetermined charging bias is applied to this charging member (a contact charging member or a contact charger, and hereinafter referred to as the contact charging member) to thereby charge the surface of the member to be charged to a predetermined polarity and potential.
The charging mechanism of contact charging (the mechanism of charging or the principle of charging) mixedly includes two kinds of charging mechanisms, i.e., (1) a discharge charging mechanism and (2) a direct injection charging mechanism, and depending on which mechanism is dominant, each characteristic presents itself.
(1) Discharge Charging Mechanism
This is a mechanism in which the surface of the member to be charged is charged by a discharge phenomenon occurring in the minute gap between the contact charging member and the member to be charged.
The discharge charging mechanism has a constant discharge threshold value for the contact charging member and the member to be charged and therefore, it is necessary to apply a voltage greater than the charging potential to the contact charging member. Also, as compared with a corona charger, it is unavoidable in principle for a discharge product to be produced though the quantity thereof produced is markedly small, and therefore the ill effects caused by active ions such as ozone is unavoidable.
For example, the charging system using an electrically conductive roller (charging roller) as the contact charging member is preferable in respect of the stability of charging and is widely used, but in this roller charging, the discharge charging mechanism is dominant as the charging mechanism thereof.
That is, the charging roller is produced by the use of an electrically conductive or medium-resistance rubber material or a foamed material. Further, there is also a charging roller of a laminated construction to thereby obtain a desired characteristic. The charging roller is given elasticity in order to obtain constant contact with the member to be charged, but therefore it is great in frictional resistance and in many cases, it is driven following the member to be charged or with some difference from the latter. Accordingly, a noncontact state is unavoidable due to the irregularity of the shape of the roller or the material adhering to the member to be charged and therefore, in the conventional roller charging, the discharge charging mechanism becomes dominant as the charging mechanism thereof.
More specifically, when a charging roller is pressed and made to abut against an OPC photosensitive member having a thickness of 25 xcexcm as a member to be charged and a charging process is carried out, the surface potential of the photosensitive member begins to rise if a voltage of about 640 V or greater is applied to the charging roller, whereafter the surface potential of the photosensitive member linearly increases with an inclination 1 to the applied voltage. Hereinafter, this threshold value voltage is defined as a charging starting voltage Vth.
That is, to obtain the surface potential Vd of the photosensitive member required for electrophotography, a DC voltage of Vd+Vth greater than required becomes necessary for the charging roller. A system of applying only a DC voltage to the contact charging member in this manner to thereby effect the charging of an image bearing member is referred to as the xe2x80x9cDC charging systemxe2x80x9d.
In the DC charging system, however, the resistance of the contact charging member is fluctuated by the fluctuation of the environment or the like and Vth is fluctuated if the film thickness is changed by the photosensitive member as the image bearing member being shaved and therefore, it has been difficult to render the potential of the photosensitive member into a desired value.
Therefore, in order to achieve the further uniformization of charging, as disclosed in U.S. Pat. No. 4,851,960, use is made of an xe2x80x9cAC charging systemxe2x80x9d of applying to a contact charging member a vibration voltage comprising an AC component having a peak-to-peak voltage of 2xc3x97Vth or greater superposed on a DC voltage corresponding to desired Vd to thereby effect the charging of an image bearing member. This is directed to the level effect of potential by AC, and the potential of the image bearing member is converged to Vd which is the center of the peak of the AC voltage, and is not affected by the disturbance of the environment or the like.
(2) Direct Injection Charging Mechanism
This is a mechanism in which charges are directly injected from a contact charging member into a member to be charged to thereby charge the surface of the member to be charged. It is proposed in U.S. Pat. No. 5,809,379, etc.
A contact charging member of medium-resistance contacts with the surface of the member to be charged to thereby effect the direct injection of charges into the surface of the member to be charged without the intermediary of a discharge phenomenon, i.e., basically without using a discharging mechanism. Consequently, even if the applied voltage to the contact charging member is equal to or less than a discharge threshold value, the member to be charged can be charged to the potential corresponding to the applied voltage. This direct injection charging mechanism is not accompanied by the production of ions and therefore does not give rise to the ill effects caused by the production of.
More specifically, this is a mechanism in which a voltage is applied to a contact charging member such as a charging roller, a charging brush or a charging magnetic brush and charges are injected into a charge holding member for a trap order or electrically conductive particles or the like of a charge injection layer lying on the surface of a member to be charged (an image bearing member) to thereby effect direct injection charging. Since the discharge phenomenon is not dominant, the voltage required for charging is only on the desired surface of the image bearing member and there is no production of ozone.
FIG. 5 of the accompanying drawings shows an example of the charging characteristics of the discharge charging mechanism described under item (1) above and the direct injection charging mechanism described under item (2) above.
That is, in the discharge charging mechanism, as represented by the graph A of FIG. 5, charging begins after a discharge threshold value of about xe2x88x92500 V is passed. Accordingly, when the member to be charged is to be charged to xe2x88x92500 V, it is popular to apply a DC voltage of xe2x88x921000 V or apply a DC charging voltage of xe2x88x92500 V, and apply an AC voltage of peak-to-peak voltage 1200 V so as to always have a potential difference of the discharge threshold value or greater to thereby converge the potential of the member to be charged to the charging potential.
On the other hand, in the direct injection charging mechanism, there is no discharge threshold value as represented by the graph B of FIG. 5, and it becomes possible to obtain a charging potential substantially proportional to the applied bias.
[Toner Recycle Process (Cleanerless System)]
In an image forming apparatus of the transfer type, any untransferred toner remaining on a photosensitive member (image bearing member) after transfer is removed from the surface of the photosensitive member by a cleaner (cleaning device) and becomes waste toner, but it is desirable from the viewpoint of environmental protection that such waste toner is not produced. So, there has also appeared an image forming apparatus of toner recycle process made into an apparatus construction in which a cleaner is eliminated and any untransferred toner on a photosensitive member after transfer is removed from the photosensitive member by xe2x80x9ccleaning simultaneous with developmentxe2x80x9d by the use of a developing device and collected and reused into the developing device for reuse.
The cleaning simultaneous with development is a method of collecting any toner remaining on the photosensitive member after transfer by continuously charging the photosensitive member during the development after the next step, exposing the photosensitive member to thereby form a latent image, and applying a fog-removing bias (a fog-removing potential difference Vback which is the potential difference between a DC voltage applied to the developing device and the surface potential of the photosensitive member) during the development of the latent image. According to this method, the untransferred toner is collected into the developing device and reused after the next step and therefore, the waste toner can be eliminated and the cumbersomeness of maintenance can be reduced. Also, the advantage in terms of space is great due to being cleanerless, and it becomes possible to make the image forming apparatus very compact.
[Application of Powder to the Contact Charging Member]
With regard to a contact charging device, a construction in which powder is applied to the surface of contact of the contact charging member with the surface of the member to be charged to prevent uneven charging and effect stable uniform charging is disclosed in Japanese Patent Publication No. 7-99442. However, although the contact charging member is rotated following the member to be charged and the production of ozone products is markedly small as compared with a corona charger such as a scorotron, the principle of charging still relies chiefly on the discharge charging mechanism as in the case of the aforedescribed roller charging. Particularly, to obtain the more stable uniformity of charging, the production of ozone products by discharge becomes more because a voltage comprising an AC voltage superposed on a DC voltage is applied.
Also, in Japanese Patent Application Laid-Open No. 5-150539, there is disclosed an image forming method using contact charging wherein a developer contains at least visualizing particles and electrically conductive particles having an average particle diameter smaller than that of the visualizing particles in order to prevent the hindrance to charging by toner particles and silica fine particles adhering to the surface of charging means while image formation is repeated for a long time.
As noted in the above-described prior art, in contact charging, powder is applied to the surface of contact of the contact charging member with the surface of the member to be charged in order to prevent uneven charging and effect stable uniform charging, but the application of this powder is difficult and has led to problems that
a) it is difficult to uniformly apply the powder to the surface of the charging member and the application of the powder becomes liable to be non-uniform, and
b) the application of the powder is uniform at the initial stage, but the powder becomes liable to be peeled by duration and becomes non-uniform.
It is an object of the present invention to provide a charging member capable of holding electrically conductive particles on the surface thereof.
It is another object of the present invention to provide a charging device capable of charging through electrically conductive particles.
It is still another object of the present invention to provide a charging member comprising an electrically conductive base material, and a surface layer including a foaming body of a continuous bubble type.
It is yet still another object of the present invention to provide a charging device comprising a member to be charged, a charging member for contacting with the member to be charged to thereby charge the member to be charged, said charging member having a surface layer including a foaming body of a continuous bubble material, and charge accelerating particles held in the bubble of the surface layer.
Further objects of the present invention will become apparent from the following description.